Most automatic transmissions for shifting gears in automobile engines comprise various sun and planetary gears with appropriate clutch plates and bands responsive to hydraulic oil pressure to hold one or more gears and thus change the effective ratio between input shaft speed from the engine and output shaft speed to the wheels. The heart of such transmission systems is a valve body containing various inter-related passages, valves, springs and orifices. The valve body receives signals in the form of hydraulic pressures which function to shift various ones of the shift valves to thereby route the transmission oil to appropriate clutch plates or bands and thus automatically effect sequential shifting within a given range. Mechanical shifting is effected by mechanical movement of other valves to change from a given shift range to another given shift range, this shifting normally being accomplished by the shift lever.
For most normal driving, there is only one position for the manual shift lever which is the drive or D position. Once the lever is placed in this position, the shifting of the gears is completely automatic. The same shift lever also serves to shift certain other valves to effect reverse movement of the automobile.
In effecting movement of the various shift valves to properly route the oil to appropriate clutch plates, the controlled hydraulic transmission oil pressure is used as described. This pressure is made responsive to the shaft speed as well as the shaft torque. The drive shaft speed is measured by a governor and this information is relayed to the valve body in the form of more or less hydraulic pressure by opening a specific valve a precise amount.
Thus, with the manual shift lever in the drive or "D" position, the automobile can be started from a rest position, the various gears automatically shifting from low through intermediate positions to high. While such shifting is completely automatic, the driver does have some control in that the manual shift lever usually includes 1 and 2 positions constituting low gear ratios as might be used when climbing a steep grade or as a braking feature when descending a steep grade. As mentioned, however, once the manual gear shift lever is set in one of its various positions, automatic shifting occurs within the given shift range of that position.
There are many instances, however, when a driver may wish to effect all shifting manually rather than have the shifting take place automatically. In the case of manual shifting, the driver would always start the vehicle in the number 1 or number 2 gear shift lever position and progress up to the drive or "D" position. In the "D" position, the gears would always be in "high" and would not automatically shift down or up with varying shaft speed and/or torque. Such manual shifting provides the driver with a maximum degree of control over the gears. Such might be desirable in certain racing car operations or in certain driving situations wherein it might be desirable to have the car remain in "low" regardless of its speed or torque requirements.
To eliminate the automatic shifting of a transmission so that the above referred to manual shifting can be effected normally would require a disassembly of the entire transmission system. Such a conversion operation is not only expensive and time consuming but in the event it is desired to convert the system back into automatic transmission shifting control, the same disassembly operation would be required.